Precise carbon control of fabricated stainless steel

ABSTRACT

A process for controlling the carbon content of fabricated stainless steel components including the steps of heat treating the component in hydrogen atmospheres of varying dewpoints and carbon potentials.

States Patent [191 Nilsen Dec. 9, 1975 PRECISE CARBON CONTROL OFFABRICATED STAINLESS STEEL Roy J. Nilsen, Pittsburgh, Pa.

Assignee: The United States of America as represented by the UnitedStates Energy Research and Development Administration, Washington, DC.

Filed: Jan. 29, 1974 Appl. No.: 438,148

Inventor:

US. Cl. 148/6.35; 148/16; 148/165;

176/78; 176/88 Int. Cl. C23C 11/12; G21C 3/34 Field of Search 148/6.35,12.1, 16, 16.7

References Cited UNITED STATES PATENTS 5/1948 Uhlig 148/16 X 3,277,14910/1966 Brickner et a1. [48/16 X FOREIGN PATENTS OR APPLICATIONS1,186,485 2/1965 Germany 148/16 118,285 2/1945 Australia 148/16 PrimaryExaminerC. Lovell Attorney, Agent, or Firm.lohn A. Horan; Richard A.Lambert 1 Claim, N0 Drawings PRECISE CARBON CONTROL OF FABRICATEDSTAINLESS STEEL BACKGROUND OF THE INVENTION This invention was made inthe course of, or under, a

contract with the United States Atomic Energy Commission.

My invention relates to a process whereby the carbon content offabricated components, particularly nuclear reactor fuel rod grids, canbe precisely and uniformly controlled to any desired level. The processwas conceived and experimentally proven feasible during development oftechniques for reducing the carbon content of AM-350 stainless steel.AM-350 is a semi-auste nitic iron-base alloy nominally containing 17percent chronium, 4 percent nickel, 3 percent molybdenum, and 0.10percent carbon. AM-350 sheet, annealed at 1900F. (Condition H) isaustenitic with small amounts of delta ferrite and is ductile and canreadily be formed into intricate components. The AM-350 composition iscarefully balanced between austenite and ferrite formers in such afashion that the solution of carbon in the austenite-ferrite matrixduring the H anneal yields a stable austenite-ferrite structure at roomtemperature. If the material is given a lower temperature L anneal(1,710F.), part of the carbon in solution precipitates. The decrease ofsolid solution carbon content decreases austenite stability and resultsin a martensitic transformation on cooling to room temperature. AM- 350processed in this fashion possesses all the desired nuclear andmechanical properties for LWBR fuel rod support systems with theexception of the degree of corrosion resistance required for corematerials. This lack of corrosion resistance is caused by grain boundarycarbide precipitates formed during the L anneal and during coolingthrough critical carbide precipitation ranges. The solution to thisproblem is to simply remove carbon in excess of that which produces adeleterious precipitate. This is most easily done during melting.However, reduced carbon AM-350 is unstable, and cannot be cold rolled orformed. Essentially any amount of deformation will cause the ductileaustenitic structure to transform a brittle martensite.

For this reason, the alloy must have a high enough carbon content forformability and after fabrication the carbon content must be reduced toimpart corrosion resistance to heat treated components. Additionally,the carbon content must not be reduced excessively or required strengthlevels will not be attainable, and the carbon content must be maintainedat an extremely uniform level to reduce nonuniform transformationinduced distortion. Carbon gradients will result in nonuniform Mstemperatures causing warpage as local areas transform before others.

Decarburization of ferrous sheet is an extremely common commercialprocess used, e.g., in the preparation of iron plus silicon alloys fortransformer cores. However, all commercial decarburization processes aredesigned to reduce the carbon content of low alloy transformer steels tothe lowest possible level to opti- C+2H2 CH,

2 The second process requires exposing components to extremely wethydrogen gas (5,000 to 10,000 ppmv H O) at temperatures in the range of1,500F. to 1,800 F. Carbon is removed by the water gas reaction Neitherof these processes is strictly applicable to the decarburization ofintricate precision components formed from chromium bearing stainlesssteel. The first process for carbon removal by the methane reactionrequires temperature far in excess of what can be tolerated from adistortion standpoint. The second process can be used but withsignificantly reduced water contents (approximately 200 ppmv H O at1,900F.), to prevent chromium oxidization With reduced water contents,the reaction is sluggish and tends to be nonuniform due to waterdepletion at localized areas. Precision carbon control is virtuallyimpossible. For these reasons, a new process for controlled carbonreduction was developed.

It was desired to reduce the as-received carbon content of an AM-350grid to precisely 0.05 weight percent (w/o). The grid is an opencellular array assembled from 0.015 inch thick components. The assemblyis approximately 20 inches in diameter, 2 inches high, and weighs 7pounds.

SUMMARY OF THE INVENTION It is an object of my invention to provide amethod of controlling the carbon content of fabricated stainless steelcomponents.

It is another object of my invention to provide a method of reducing thecarbon content of fabricated stainless steel components.

It is another object to provide a method of control-' ling the carboncontent of nuclear reactor fuel rod grids to a precise, preselectedvalue.

Other objects of my invention will become apparent from the followingdescriptions and the attached claims.

In accordance with my invention I have provided a method of controllingthe carbon content of fabricated stainless steel components comprising:(.a) decarburizing the component in two steps;

1. heating the component to an oxidizing wet hydrogen atmosphere to formchromium oxide on the surface of the component,

2. replacing the wet hydrogen with dry hydrogen to prevent furtherformation of chromium oxide and to permit the reduction of chromiumoxide by carbon (autodecarburization); and optionally b. carburizing thedecarburized component to the desired level by equilibrating in reducingC-O-H mixtures.

These steps are explained in detail below, with particular reference toAM-350 stainless steel fuel rod grids.

DETAILED DESCRIPTION AND EXAMPLE A. Decarburization is achieved byexposing the fabricated AM-350 stainless steel component to an oxidizingatmosphere of wet hydrogen having a dew point of from 23F. to -7F. at atemperature of from l,880F. to l920F. for about 1 hour. Preferably thecomponent is exposed to a hydrogen atmosphere containing approximately2,000 ppmv water for 28 to 40 minutes at a temperature of about 1,900F.(The actual water concentration is not critical, provided it is inexcess of 200 ppmv at 1,900F.) Three reactions occur and are listed Asindicated, some decarburization occurs during this step as a result ofthe water gas reaction (2) and the formation of hydrocarbons (3).However, the most significant reaction is the formation of Cr O on theAM- 350 surface.

It was mentioned earlier that a similar process is used to decarburizelow alloy steel, but that it was not suitable for stainless steelbecause chromium was oxidized. My invention, however, actually takesadvantage of chromium oxide formation in the followingautodecarburization step.

B. Purge the oxidizing H O H mixture with dry hydrogen (dew point 80F.or less) to prevent further formation of Cr O and maintain the componentat a temperature of 1880F. to 1920F. Preferably the component ismaintained at a temperature of 1900F. for to minutes. Two reactionsoccur and are listed below in order of significance:

2 3 surface 3 m: mhnlnn 2 Cr+ 3 CO mllll xulullun 2 4 This step of theprocess results in a thorough decarburization of the charge.Decarburization (autodecarburization) is effected by the reduction of CrO by carbon in solid solution in the AM-35O base metal. Some Cr O willbe reduced by hydrogen; however, the kinetics of this reaction are slowcompared to Cr O reduction by carbon. Reduction of Cr O by hydrogenproduces water which may contribute to some further decarburization bythe water-gas reaction (2). Additionally, some carbon is lost by theformation of hydrocarbons (5). Carbon levels as low as 0.05 w/o havebeen obtained with my method.

The carbon content of the component has now been reduced to a very lowlevel and can be used in applications where corrosion resistance isparamount and strength is of little importance. However, generally it isdesirable to raise the carbon content to a level great enough to impartadequate strength to the component. The following carburizing step willalso be explained with particular reference to AM-35O stainless steel.

The final step of the process requires equilibrating the charge inreducing C O H mixtures exhibiting a carbon activity equal to anequilibrium AM-350 carbon concentration of 0.05 w/o at 1,900F.

Virtually any gas mixture of the desired carbon potential can be usedfor equilibration; however, CH and H and CO, H 0, and H mixtures havebeen used experimentally with good success. Atmospheres containingnitrogen should be avoided since nitrogen pickup can have deleteriouseffects on stainless steel mechanical properties. At equilibrium thefollowing reversible reactions occur for CH and H and CO, H 0, and Hmixtures, respectively.

Rewriting these equations in terms of equilibrium constants yields:

4 Inserting carbon activities for carbon steel, and the equilibriumconstants for equations 6.A and 7.A from Composition of AtmospheresInert to Heated Carbon Steel R. W. Gurry, Trans Aime Vol. 188, 4-1950,p. 671, yields:

.018 CO 7.C .126

Equation 6.C when solved indicates that a hydrogen atmosphere containing31 ppmv CH has a carburizing potential of 0.05 w/o for the AM-350composition at l,900F. Equation 7.C cannot be solved directly withoutfirst selecting a moisture content. At 1900F. the equilibrium partialpressure of water for the oxidization of chromium, Equation 1, isapproximately 200 ppmv. It was found experimentally, that thecarburization reaction occurs most rapidly at this moistureconcentration, and is the basis for selecting this concentration.Equation 7.C indicates that a hydrogen atmosphere containing 105 ppmv COand 200 ppmv H O also has a carburizin g potential of 0.05 w/o for theAM-350 composition at 1,900F.

One variation of the process which may substantially reduce the lengthof the cycle would be to combine the purging and equilibration steps.Rather than first reducing the overall carbon level to less than 0.05w/o and then increasing the level to 0.05 w/o one could establish afloor, e.g., 0.05 w/o below which the carbon content could not bereduced. This can be accomplished by exposing the charge after oxidationto a purging and equilibrating hydrogen mixture of the same CO/I-I Oratio (105/200 but containing less than 200 ppmv H O. This gas mixturecould be, e.g., 52.5 ppmv CO, ppmv H 0, and hydrogen.

Theoretically, if an AM-350 structure was exposed to either a hydrogenatmosphere containing 31 ppmv CH or ppmv CO and 200 ppmv H O, for longtimes, the resulting carbon content would be precisely 0.05 w/o. Inpractice, however, this was found not to be the case. In order to obtainan equilibrium carbon concentration of 0.05 w/o using H CH it wasnecessary to use approximately 300 ppmv CH This discrepancy can beattributed to flow meter errors, and to impurities in the hydrogen gaswhich could oxidize the methane prior to exposure to the AM-350 asfollows:

8.2CH4+O22CO+4H2 9. CH H2O C0 H For this reason the H CH gas mixturewhile it can be used, is not recommended.

However, much better experimental agreement with theoreticalcalculations was found when using H CO H O mixtures. For example,hydrogen atmospheres containing 350 ppmv CO and 200 ppmv H O willconsistently yield an average carbon content of 0.054 w/o, afterequilibration at 1900F. for 1 hour. Errors causing this discrepancy mustinclude uncertainties in water content determination, and flow metercalibrations. and uncertainties in factors used to compute carbonactivity in AM-350.

As mentioned, the initial oxidizing treatment, decarburizationdeoxidizing treatment (autodecarburization) and equilibration treatmentswere carried out each for 1 hour at 1,900F. These times and temperaturesare conservative and were chosen based on calculations that thediffusion time for a carbon atom to traverse one-half the 0.015 inchAM-3 50 thickness at 1900F. is approximately 1 hour. Times andtemperatures could be adjusted for different materials and thicknesses,and to increase or decrease the reaction rates. The carbon content ofthe entire cross section of every component treated in this fashion canbe con trolled to 0.05 w/o or virtually any other carbon level desired.Increasing the desired carbon level would simply require increasing theCO content of the hydrogen gas as necessary; the opposite would be truefor lower carbon contents. Thicker gauge components would require longertimes at temperature or increased temperatures. Additionally, Steps Aand B alone can be used to decarburize chromium bearing stainlesssteels.

EXAMPLE It was desired to reduce the as-received carbon content (about0.10 w/o) of an AM-350 grid to about 0.05 w/o. The grid is an opencellular array assembled from 0.015 inch thick components. The assemblyis approximately 20 inches in diameter, 2 inches high, and weighs 7pounds.

The grid was exposed to an oxidizing H O H mixture containingapproximately 2,000 parts per million 6 by volume (ppmv) for 1 hour at1,900F. Some decarburization occurred during this step as a result ofthe water gas reaction and the formation of hydrocarbons, however, themost significant reaction was the formula tion of Cr O on the AM-35Osurface.

The oxidizing H O H mixture was purged with dry hydrogen (less than 200ppmv H 0) and the grid was maintained in this atmosphere for about 10minutes at a temperature of l,900F. The grid was found to have a carboncontent of about 0.005 w/o.

The dry hydrogen was then replaced with a H CO H O mixture containing350 ppmv CO and 200 ppmv H O. The grid was maintained in this atmospherefor 1 hour at l,900F. After this treatment the grid was found to have acarbon content of 0.054 w/o.

I claim:

1. A method for controlling the carbon content to a level of about 0.05weight percent in a fabricated AM- 350 stainless steel nuclear reactorfuel rod grid-com prising:

a. exposing the said nuclear reactor fuel rod grid to an oxidizingatmosphere of wet hydrogen having a dew point of from 23F. to 7F. at atemperature of from 1,880F. to 1,920F. for a period of 28 to 40 minutes;

b. replacing said oxidizing atmosphere with dry hydrogen having a dewpoint of F. or less and maintaining said nuclear reactor fuel rod gridin said dry hydrogen for a period of 10 to 15 minutes at a temperatureof 1,880F. to 1,920F.; and

. c. replacing said dry hydrogen atmosphere with a reducingcarbon/oxygen/hydrogen atmosphere containing about 350 ppmv CO and about200 ppmv H 0 and exhibiting a carbon activity equal to an equilibriumAM-3 50 carbon concentration of about 0.05 weight percent at about1,900F and maintaining said grid at said temperature for about 1 hour.

1. A METHOD FOR CONTROLLING THE CARBON CONTENT TO A LEVEL OF ABOUT 0.05WEIGHT PERCENT IN A FABRICATED AM-350 STAINLESS STEEL NUCLEAR REACTORFUEL ROD GRID COMPRISING: A. EXPOSING THE SAID NUCLEAR REACTOR FUEL RODGRID TO AN OXIZING ATMOSPHERE OF WET HYDROGEN HAVING A DEW POINT OF FROM:23*F TO -7*F. AT A TEMPERATURE OF FROM 1,880*F. TO 1,920*F. FOR APERIOD OF 28 TO 40 MINUTES, B. REPLACING SAID OXIDIZING ATMOSPHERE WITHDRY HYDROGEN HAVING A DEW POINT OF -80*F. OR LESS AND MAINTAINING SAIDNUCLEAR RACTOR FUEL ROD GRID IN SAID DRY HYDROGEN FOR A PERIOD OF 10 TO15 MINUTES AT A TEMPERATURE OF 1,880*F. TO 2,920*F, AND C. REPLACINGSAID DRY HYDROGEN ATOMPHERE WITH A REDUCING CARBON/OXYGEN/HYDROGENATMOSPHRE CONTAINING ABOUT 350 PPMV CO AND ABOUT 200 PPMV H2O ANDEXHIBITING A CARBON ACTIVITY EQUAL TO AN EQUILIBRIUM AM-350 CARBONCONENTRATION OF ABOUT 0.05 WEIGHT PERCENT AT ABOUT 1,900*F ANDMAINTAINING SAID GRID AT SAID TEMPERATURE FOR ABOUT 1 HOUR.